Active Structural Control for a Smart Guitar & Violin

Introduction and Background

In 1990, the "Mendelssohn" Stradivarius violin sold at Christie's in London for $1,686,700 while a good violin at a typical music store sells for abour $2,000 or $2,500. Then, a natural question that one may ask is as follows. Why does a Stradivarius cost so much more than an ordinary violin? The goemetry of the musical instruments is very similar. However, in addition to the prestige associated with a Stradivarius, there are subtle differences in the structural dynamic characteristics of the two musical instruments. These subtle differences result in yielding different dynamic responses of these musical instruments when they are excited by the same accomplished violinist. The different dynamic response or the different vibrations of the two instruments determine the quality of the acoustic radiations, hence the quality of the music and the associated price.

Then, a natural curiosity by a structural dynamicist is to explore methods of changing and improving the structural dynamic design of an ordinary violin to realize the structural dynamic characteristics of the Stradivarius. Many instrument designers whether they are structural dynamic engineers or simply a violinmaker without the background of physics and engineering, have explored the possibility of changing the material and design to obtain the legendary quality of the sound radiated by a Stradivarius. Some designers thought it was the quality of the wood. Others thought it was the procedure used to cut the wood or the type of finish used by the famous craftsman Stradivari. From the piont of view of a structural dynamicist, the wood, seasoning methods, the method of manufacturing or the types of finish only change the structural dynamic characteristics like the natural frequencies, modes, damping and possible nonlinear effects. These characteristics can also change from the design to the manufacturing process.

Then, the next question is as follows. Is it possible to force the less expensive violin to vibrate like a Stradivarius by using active structural control techniques and thus produce the legendary quality of the music that can now be produced by a Stradivarius? If we can design and implement such an active controller, the less expensive violin can produce acoustic radiations, and hence the quaility of the music, like a good quality or an expensive violin.

Our initial efforts were to achieve such an objective by the use of smart structural/acoustic control techniques. The field of smart structural control is centered upon changing the structural dynamic characteristics of a structure by active techniques. In an active controller, modification of characteristics is accomplished by the use of piezoceramic actuators and sensors integrated into a structural system. These actuators are activated by applying an electric field that is determined by a controller and a sensor, not to amplify the sound, but to change the acoustic characteristics. In our laboratories, the research has been implemented in a store-bought guitar to produce a good quality acoustic guitar.

By the use of smart structural/acoustic control techniques, we design a "smart sound post" for a violin by using piezoceramic actuators integrated into the wood piece of sound post, to change the acoustic characteristics. In our laboratories, the research has been implemented in a store-bought violin to produce a good quality violin.